The present invention concerns a process and apparatus for accurately controlling the rate of introduction and the content of alumina in an igneous electrolysis tank, and the use thereof for the production of aluminium using the Hall-Heroult process.
In recent years, operation of aluminium production tanks has been progressively automated, both in order to improve the power balance sheet and operating regularity thereof and also to limit the extent of human involvement and improve the efficiency in collecting the fluorinated effluents.
One of the essential factors in regard to ensuring operating regularity of a tank for the production of aluminium by the electrolysis of alumina dissolved in molten cryolite is the rate of introduction of the alumina into the bath. An alumina deficiency causes the occurrence of the `anodic effect` or `racing` phenomenon, which causes an abrupt increase in the voltage at the terminals of the tank, which can go from 4 to 30 or 40 volts, and which has repercussions on the entire production process.
An excess of alumina gives rise to the danger of the bottom of the tank being fouled by alumina deposits which may be converted into hard plates which electrically insulate a part of the cathode. This induces the generation of very strong horizontal currents in the metal of the tanks. By interaction with the magnetic fields, such currents agitate the layer of metal and cause instability at bath-metal interface.
This defect is particularly troublesome when it is desired to lower the operating temperature of the tank, which is highly advantageous in regard to the level of Faraday efficiency, by adopting highly `acid` baths (with a high AlF.sub.3 content) or baths which include various additives such as lithium or magnesium salts or chlorides. However, these baths suffer from a substantially reduced capacity and alumina dissolution speed, and the use thereof involves very accurately controlling the alumina content, to relatively low levels of concentration and between two relatively close limit values.
Although it is possible for the alumina content of the baths to be measured directly by analyzing samples of electrolyte, for many years now the method selected has been to effect indirect evaluation of the alumina content by following an electrical parameter which reflects the concentration of alumina in the electrolyte.
This parameter is generally the variation in internal resistance or, more precisely, the internal pseudo-resistance which is equal to: ##EQU1## in which e is an image of the back e.m.f. of the tank, whereby it is generally accepted that it is 1.65 volts, U is the voltage at the terminals of the tank, and I is the amperage of the current passing therethrough.
By taking a series of readings, it is possible to trace a curve showing the variation in R relative to the alumina content, and by measuring R at a given frequency, using methods which are well known at the present time, it is possible at any moment to ascertain the level of concentration as symbolically represented by [Al.sub.2 O.sub.3 ].
For many years now, attempts have been made to introduce alumina into the bath with a certain degree of regularity so as to maintain the level of concentration thereof relatively stable about a predetermined value.
Processes for the automatic feeding of alumina, which are controlled more or less strictly relative to the concentration of alumina in the bath, have been described in particular in the following patents: French patent No. 1 457 746 to Reynolds, wherein the variation in internal resistance of the tank is used as a parameter reflecting the level of concentration of alumina, which is introduced into the bath by means of a distributor combined with a means for making a hole in the crust of solidified electrolyte; French patent No. 1 506 463 to V.A.W. which is based on measuring the time which elapses between stopping the feed of alumina and the occurrence of the anodic effect; U.S. Pat. No. 3,400,062 assigned to ALCOA which uses a `pilot anode` in order to achieve advance detection of `racing` and control the rate of introduction of the alumina which is distributed from a hopper provided with a means for making a hole in the crust of solidified electrolyte.
The alumina feed means is described in greater detail in U.S. Pat. No. 3,681,229 which is also assigned to ALCOA.
More recently, control processes based on monitoring the alumina content have been described in particular in Japanese patent application No. 52-28417/77 to SHOWA DENKO, and in U.S. Pat. No. 4,126,525 assigned to MITSUBISHI.
In the first of these patents, the level of concentration of alumina is fixed in the range of from 2 to 8%. The variation .DELTA.V in dependence on time t, in the voltage at the terminals of each tank is measured, and is compared to a predetermined value. The rate at which alumina is introduced is modified to adjust the .DELTA.V/T to the standard value. The disadvantage of this process is that the sensitivity thereof varies with the alumina content which is actually at a minimum in the range used, from 3 to 5% of Al.sub.2 O.sub.3 (see the Table on page 84).
In the second of these patents, the alumina content is also fixed in the range of from 2 to 8% and preferably from 4 to 6%. The tank is supplied for a predetermined period of time t.sub.1, with an amount of alumina which is higher than the theoretical consumption thereof, until a predetermined level of concentration of alumina is obtained (for example up to 7%), then the feed is switched over to a rate equal to the theoretical consumption, for a predetermined period of time t.sub.2, the feed is then stopped until the initial symptoms of anodic effect (`racing`) appear, and the feed cycle is resumed at a rate which is higher than the theoretical consumption.
In this process, the concentration of alumina varies from 4.9 to 8% (Example 1) or from 4.0 to 7% (Example 2), in the course of the cycle.
These different processes lack accuracy and do not solve the problem set, which is that of controlling the content of alumina between narrow limits.